The inventive subject matter relates to nonvolatile memory devices and, more particularly, to variable resistance nonvolatile memory devices.
Examples of nonvolatile memories using variable resistance materials include phase-change random access memories (PRAMs), resistive RAMs (RRAMs), and magnetic RAMs (MRAMs). While dynamic RAMs (DRAMs) or flash memories store data using charges, nonvolatile memories using resistance materials store data using a state change of a phase-change material such as chalcogenide alloy (as in the case of PRAMs), a resistance change of a variable resistance material (as in the case of RRAMs), or a resistance change of a magnetic tunnel junction (MTJ) thin film according to a magnetization state of a ferromagnetic material (as in the case of MRAMs).
For example, a phase-change material of a PRAM cell takes on a crystalline state or an amorphous state as it is cooled after being heated. Typically, the phase-change material has low resistance in the crystalline state and has high resistance in the amorphous state. Therefore, the crystalline state may be defined as “set” data or data “0”, and the amorphous state may be defined as “reset” data or data “1”.
A read circuit which reads data stored in a phase-change memory cell may include a clamping element. The clamping element maintains a phase of the phase-change memory cell during a read operation. In particular, the clamping element maintains a read current substantially constant while preventing the phase-change memory cell from having a voltage higher than a predetermined voltage. A conventional clamping element may use an n-channel metal oxide semiconductor (NMOS) transistor and may clamp the voltage of the phase-change memory cell to a voltage level equal to a difference (i.e., Vcmp−Vth) between a gate voltage Vcmp and threshold voltage Vth of the NMOS transistor. However, since the threshold voltage Vth varies according to process and temperature, the clamping operation may not be stable.
A typical read circuit reads data by providing the read current to the phase-change memory cell. To generate the read current, however, a high-level step-up voltage Vpp, generated by pumping a power supply voltage, is typically used. The pumping circuit that generates the step-up voltage Vpp may have a low current efficiency and may occupy a large area.